So-called Active Antenna System (AAS) base stations (sometimes also referred to as Advanced Antenna System base stations) have become a topic of increasing interest in recent years. An AAS base station may have some or all of the electronic components of a radio frequency (RF) transceiver integrated with an antenna unit, and may have multiple transceiver chains. An AAS base station may have the ability to perform electronic beam forming on transmitted and received signals.
FIG. 1 illustrates a reference architecture for an AAS base station. More particularly, FIG. 1 illustrates various components of an AAS base station, including antenna array 10, radio distribution network (RDN) 15, radio transceiver array 20, and baseband processing unit 25.
Antenna array 10 may include any suitable antenna elements. For example, antenna array 10 may include any suitable number of antennas. Antenna array 10 may have any suitable arrangement. For example, there exist a number of potential physical arrangements for antenna array 10, which may include (but are not limited to) uniform linear, matrix and circular. Typically, cross polarized arrangements are deployed with an antenna element for each polarization.
RDN 15 may be responsible for routing of RF signals between each transceiver output and one or more antennas. In some cases, RDN 15 may be as simple as a direct coupling from each transceiver to each antenna. In other cases, RDN 15 may be a complex network of feeders and filters, with each transceiver driving multiple antenna elements.
Radio transceiver array 20 may include any suitable number of transceivers. Transceivers of radio transceiver array 20 may contain transmit chains and receive chains. Transmit chains may contain typical components such as filters, mixers, power amplifiers (Pas), and any other suitable components. Receive chains may contain typical components such as filtering, low noise amplifiers (LNAs), and any other suitable components. In some cases, the number of transmitters may not be equal to the number of receivers. Baseband processing unit 25 may perform the processing functions of the AAS reference architecture. In certain embodiments, baseband processing unit 25 may perform electronic beam forming.
As described above, AAS base stations may have the ability the ability to perform electronic beam forming on transmitted and received signals. Beam forming may be performed at various places within the architecture. For example, beam forming may be performed in baseband processing unit 25, or may be performed as part of the transceivers of transceiver array 20, or RDN 15 may implement analogue beam forming. There may be several approaches to beam forming, depending on the type of base station architecture and the intended application. Broadly speaking, beam forming types can be split into two categories: cell specific beam forming and user specific beam forming.
Cell specific beam forming is beam forming that is performed on signals that are common to all user equipments (UEs), in order to impact the footprint of a cell. Examples of cell specific beam forming include variable electronic downtilt, cell splitting, and cell shaping. In variable electronic downtilt beam forming, the elevation angle of the main lobe of the cell coverage beam is tilted downwards by means of electronic beam forming, typically to reduce downlink intercell interference. Downtilt is achievable using non AAS, passive antenna systems. AAS systems, however, offer new possibilities, such as varying the downtilt according to traffic conditions, and applying different degrees of downtilt to different carriers and radio access technologies (RATs).
In cell splitting, beam forming is used to create beams with different cell IDs. Cell splitting enables a more adaptive approach to increasing capacity. Cell splitting may also be applied differently on different carriers or RATs. In cell shaping, the coverage pattern of a cell is made irregular by means of beam forming. For example, cell shaping may be applied in heterogeneous networks to adjust the coverage area of a macrocell to avoid interference to a microcell.
User specific beam forming is beam forming that is applied only to resources scheduled for a particular user. User specific beam forming is typically dynamic and dependent on channel conditions and scheduler decisions. Examples of user specific beam forming include non-coherent beam forming, coherent beam forming, and uplink beam forming. Non-coherent beam forming refers to a situation in which the phase between multiple antennas is not predictable. Typically, in the downlink the UE will send feedback information to the base station relating to the instantaneous receive signal conditions from the beam forming antennas, such that the base station can apply instantaneously optimal beam forming. Non-coherent beam forming may be used to achieve spatial multiplexing of multiple streams to a single user, or multiple streams to multiple users.
In coherent beam forming, the difference in phase that the UE experiences from different antennas is constant and known in the base station. By means of applying a so-called “phase progression” (and optionally, amplitude weights) to the signal transmitted to each antenna, a base station can create a narrow beam width beam that is directed towards a specific user. This will both increase the received signal to the user in question and decrease interference to other users. Coherent beam forming can be used to spatially multiplex streams to different users that are geographically separated. Uplink beam forming typically aims to maximize receiver SINR for an uplink user by means of maximizing combining gain for the signal from the user in question whilst steering spatial nulls towards significant sources of interference.
FIG. 2 illustrates the difference between user specific beam forming and cell specific beam forming. More particularly, FIG. 2 illustrates a plurality of UEs 110A, 110B, 110C, network node 115, and a plurality of coverage areas 130, 140, 150A, 150B. UEs 110A-C may be any suitable wireless device, and may communicate with network node 115 over a wireless interface. For example, UE 110A may transmit wireless signals to network node 115 and/or receive wireless signals from network node 115. The wireless signals may contain voice traffic, data traffic, control signals, and/or any other suitable information. In some embodiments, an area of wireless signal coverage associated with a network node 115 may be referred to as a cell.
Network node 115 may have the ability to perform electronic beam forming. For example, network node 115 may be an AAS base station. Network node 115 may interface with one or more components of a network. For example, network node 115 may interface with a radio network controller. The radio network controller may control network node 115 and may provide certain radio resource management functions, mobility management functions, and/or other suitable functions. The radio network controller may interface with a core network node. In certain embodiments, the radio network controller may interface with a core network node via an interconnecting network. The interconnecting network may refer to any interconnecting system capable of transmitting audio, video, signals, data, messages, or any combination of the preceding.
In some embodiments, the core network node may manage the establishment of communication sessions and various other functionality for UEs 110A, 110B, and 110C. UEs 110 may exchange certain signals with the core network node using the non-access stratum layer. In non-access stratum signaling, signals between UEs 110 and the core network node may be transparently passed through a radio access network. Example embodiments of UEs 110, network node 115, and other network nodes (such as the radio network controller or core network node) are described with respect to FIGS. 11, 12, and 13, respectively.
UEs 110A-C may transmit wireless signals to network node 115 and/or receive wireless signals from network node 115. Coverage areas 130, 140, 150A and 150B illustrate the coverage areas in which UEs 110A-C may transmit and/or receive wireless signals from network node 115. For example, coverage area 130 illustrates the coverage of physical antenna elements of network node 115. In some cases, the size of coverage area 130 may be larger than the area intended to be covered by network node 115. Furthermore, coverage area 130 may result in interference. Coverage area 140 illustrates the coverage area of cell-specific beams. Coverage area 140 may represent the area in which UEs 110A-C may detect network node 115, and perform radio measurements. Coverage areas 150A and 150B illustrate the coverage of UE specific beams. For example, coverage area 150A may be a UE specific beam for UE 110A. Similarly, coverage area 150B may be a UE specific beam for UE 110B.
There exist a variety of deployment strategies for AAS base stations. One is a typical macro deployment in which the above mentioned beam forming techniques can be used for improving capacity (e.g., by means of spatial multiplexing), reducing interference (e.g., by means of variable downtilt), or enabling offloading between cells (e.g., by means of using user specific beam forming to improve SINR when trying to serve users located in neighbor cells).
FIG. 3 illustrates a scenario in which user specific beam forming is used for macro offloading. More particularly, FIG. 3 illustrates a plurality of cells 310A-G. Each cell 310 may have an associated coverage area, and may include one or more network nodes, such as network node 115 described above. For example, cell 310C may be a macro cell having a macro node. Some of the one or more network nodes associated with each cell 310 may be AAS capable network nodes, and therefore may be able to perform electronic beam forming. For example, cell 310G may include an AAS capable network node.
A UE, such as UE 110A, may be located within one of the cells, such as cell 310C. Normally, UE 110A located in cell 310C would be served by a network node in cell 310C. In certain circumstances, however, the cell in which UE 110a is located may be overloaded. In such circumstances, a neighboring cell, such as cell 310G, may have additional capacity, and may include an AAS capable network node. The AAS capable network node of cell 310G, therefore, may be able to perform user specific beam forming to UE 110A, despite UE 110A being located in cell 310C. In such circumstances, UE 110A may be handed over to cell 310G, which has additional capacity. As a result, the load on cell 310C may be decreased.
AAS is also an interesting technology for deployment in so called heterogeneous networks. In heterogeneous networks either macro calls, or small cells (e.g., femto cell, pico cell, micro cell, etc.), or both may contain AAS base stations. One scenario that is of particular interest is that of a heterogeneous network in which the small cells (e.g., medium range or micro Node Bs or eNode B) are deployed to offload traffic from the macro network. In such situations, it may be the case that it is beneficial to serve some UEs from a small base station, even though they are far enough away from it that the macro node has the strongest receive signal (e.g. RSRP, RSRQ, CPICH Ec/Io, CPICH RSCP). In general, although these UEs will experience poorer instantaneous throughput due to interference from the macro cell, offloading them may enable the low power node (LPN) to serve the UEs more often, thus increasing their mean throughput whilst further offloading the macro cell.
FIG. 4 illustrates a scenario in a heterogeneous deployment. More particularly, FIG. 4 illustrates a macro node 115A and a LPN 115B. As described above, in a heterogeneous network a UE outside of a traditional cell coverage area of a low power node may be served by the low power node. Coverage area 400 illustrates the coverage area associated with macro node 115A. Coverage area 410 illustrates the traditional coverage area of LPN 115B. Coverage area 420 illustrates the coverage area that can be achieved at LPN 115B using cell range extension or cell range expansion (CRE). One reason that CRE may be deployed is that, due to different transmit (TX) power for macro node 115A and LPN 115B, at the LPN cell border the uplink (UL) path loss to LPN 115B is much lower than that to macro node 115A even though the downlink (DL) receive signal levels are similar. In such a case, it may be preferable to serve a UE, such as UE 110 described above, from LPN 115B so that UL power control can be performed from LPN 115B. Another reason is that at least some of the load of macro node 115A can be offloaded to LPN 115B. This may result in better overall throughput.
FIG. 5 illustrates low power node cell range extension due to unequal uplink path loss. Normal cell border 510 illustrates the typical cell border based on the DL. Extended cell border 520 illustrates the extended cell border based on UL when cell range extension is used. In certain circumstances, a UE operating within the scenario illustrated in FIG. 5 may be served by macro node 115A. Such a UE may need to transmit to macro node 115A, which may result in interference in the UL to LPN 115B. In order to minimize the interference, the UE may be served at LPN 115B to the extended cell border based on UL.
AAS has the potential to improve the performance of scenarios in which cell range extension is performed. For example, user specific beam forming can be used to increase the SINR for UEs within the cell range extension zone whilst not increasing the interference observed by other users in the system.
Generally, a UE, such as UE 110 described above, may be handed to an LPN, such as LPN 115B described above, in a cell range extension zone when it is in an active connected state such as long term evolution (LTE) ACTIVE mode or high speed packet access (HSPA) CELL_DCH state. In order to make a decision to hand over the UE to an LPN or macro, a radio resource control (RRC) or radio resource management (RRM) unit will take into account measurement reports from the UE. The UE reported measurement results may be a measurement result or an event, which in turn is triggered by the UE based on the measurement result and one or more thresholds. For LTE, measurement reports may relate to common reference symbol (CRS) based reference signal received power (RSRP) or CRS based reference signal received quality (RSRQ). For HSPA, measurement reports may relate to common pilot channel (CPICH) based received signal code power (RSCP) or CPICH based chip energy to noise ratio (Ec/No). In either case, the measurements are made on fixed power, cell wide pilots or reference signals transmitted by the LPN that do not experience user specific beam forming.
Thus, decisions on handover when the UE is in an active state may be made based on measurement reports from the UE that are made on the cell wide pilot. In traditional passive antenna systems, the amount of antenna gain experienced by the cell wide pilot is the same as the antenna gain experienced with user specific signals. With AAS systems that are capable of user specific beam forming, however, user specific beams may experience a higher effective antenna gain than the common signals, such as the RS/CRS/pilots. Thus, when considering handing over users to an LPN or another macro cell for offloading, RRM will tend to underestimate the achievable throughput and offloading. This will lead to suboptimal usage of radio resources, since in some circumstances a UE would not be offloaded to another cell due to poor RS/CRS/pilot based measurements, even though with user specific beam forming an acceptable user throughput could be achieved from the other cell. Since the user specific beam forming gain is specific to the AAS architecture and application, it is not predictable for the source base station to guess the potential user specific beam forming gain offered by another base station.